This invention relates to a method of brightness-dependent control of a lamp used, more specifically, for illuminating an LCD (Liquid crystal display). LCD's are preferably used in quartz clocks and pocket calculators. As liquid crystal displays allow of a substantially powerless operation and consequently require only a very low power supply, they are also increasingly used in devices used in the field of communication.
Liquid crystal displays are passive displays, that is to say they control the light incident on them. Liquid crystal displays are provided between front and rear polarizers with a layer of "twisted nematic liquid crystals". Ignoring heat fluctuations, the longitudinal axes of the elongate molecules are parallel to each other in the nematic phase. In response to an electric field applied to the electric crystal display, the originally opaque liquid crystal layer becomes transparent.
If no external electric field is applied, then the direction of the axis of each molecular layer is rotated through a small angle with respect to the adjacent molecular layer. If, for example, the light (daylight illumination or light from a lamp) enters from the rear with a horizontal polarization, then this light is vertically polarized on leaving the liquid crystal layer. If the polarizer arranged at the front of the LCD also has a vertical orientation, then the light travels unimpeded through this polarizer and, for the observer, the picture area makes the impression of being bright.
Liquid crystal displays can be divided into reflective displays, that is to say light incident from the front is reflected or absorbed depending on the switching state of the picture elements, a transmissive display, that is to say light entering from the rear is controlled in correspondence with the information to be displayed and a transflective display which has a semipermanent reflector possessing the properties of the transmissive and the reflective displays. In the negative contrast display mode the information is displayed in bright characters on a dark background, whereas in the positive contrast display mode the representation of the information is effected by dark characters on a bright background.
In liquid crystal displays the liquid crystal layer is sandwiched between front and rear glass plates and a seal is applied along the edges between the glass plates. Transparent electrodes are provided on the glass plates so that when a voltage is applied to the two electrodes, the liquid crystal layer is exposed to an electric field. This electric field causes all the molecule layers to be aligned with the electric field, as a result of which the twist in the electric crystal layer and consequently the rotation of the direction of polarization of the incident light disappears. If light of a horizontal polarization enters from the rear, this light passes with a horizontal polarization at the front so that a vertically oriented polarizer thereat prevents the light from passing it. The picture area strikes the observer as being dark.
For the display of characters, graphs and images, a matrix display is often used. Such a matrix display has horizontal and vertical picture elements which are always arranged in parallel with each other. To display a set of specific characters, for example numbers, a segmented display, for example a seven-segent display is often used. For a direct drive during addressing or selection, respectively, of the picture elements and their brightness control circuit. If each segment or each picture element (pixel) is directly driven, a large number of driver circuits and feeder lines are required therefor. A liquid crystal display for, for example, a five-digit number each having seven segments requires 35 control conductors and a common return conductor, when each segment is to be individually driven. A.c. voltages having a frequency exceeding 30 Hz are used to drive the liquid crystal display in order to obtain a flicker-free image.
To reduce the number of feeder lines and drive circuits, liquid crystal displays are controlled in the multiplex mode, that is to say that, for example, each column or line is activated time-sequentially. The voltage amplitude difference between the voltage across energized and non-energized segments (or picture elements) decreases for an increasing multiplex ratio, the threshold voltages of energized and non-energized picture elements not being distinct and, in addition, varying as a function of temperature. For high multiplex ratios a correspondingly steep control characteristic is consequently required, a pulse amplitude modulation often being used for brightness control of the picture elements, that is to say the amplitude of the drive pulses determines the permeability to light and consequently the brightness of the picture element.
As the switching mechanism in the liquid crystal layer is based on a thermal effect, the heat generation in the liquid crystal display must be kept within limits. Due to incident sunlight, ambient illumination levels up to 10.sup.5 lux may occur so that the liquid crystal display is difficult to read.
To improve the readability it is known, for a liquid crystal display with positive contrast display, to place a light source at the front face of the liquid crystal display (DE-PS 23 10 219).
In addition, DE-PS 30 29 122 discloses a liquid crystal display arrangement for a motor vehicle. The transmissive display is provided at its rear side with a luminescent body, which for the optional introduction of the ambient light or the light of a light source into the liquid crystal layer is of a movable structure.
In the daylight mode the light source is in the switched-off state and the daylight is concentrated by means of a lens and illuminates the luminescent body. The luminescent body has a reflecting coating at its rear side, through which the incident daylight illuminates the liquid crystal layer from behind.
In the night mode the luminescent body is located at the rear of the liquid crystal display device and the light source is switched on. The light from the light source, reflected from the reflecting coating of the luminescent body is now used to illuminate the liquid crystal display device.
Switching on the light source and simultaneous motor-driven readjustment of the luminescent body to the night mode is effected by means of a switch which is operable on switch-on or switch-off of the lights of the motor vehicle. To prevent manual operation of a switch it is possible to use an electric switch which, in dependence on the brightness (a brightness measured, for example, by means of a cadmium sulphide photo sensor) of the incident ambient light switches on the light source and the motor for readjustment of the luminescent body. It is indeed true that then over-heating of the liquid crystal display device by heat radiation from the light source is prevented, but on the other hand the readjustment of the luminescent body requires a higher design effort and cost.